1. Field of the Invention
The present invention relates to a developing device for image forming apparatuses, which is used in image forming apparatuses such as xerographic copiers, laser printers and facsimile machines which form images by development using a non-magnetic, one-component type developer, and which comprises, all received in a developer tank: a developing roller for supplying a developer to a photoconductor; a developer feed roller for conveying and supplying the developer to the developing roller, placed in non-contact with the developing roller; and a toner (or developer)-layer thickness control member which controls the thickness of the developer layer on the developing roller.
In addition, the present invention relates to a developing device which develops an electrostatic latent image formed on a recording medium, wherein a one-component type developer is conveyed to the developing region while being applied to the developing roller at a constant thickness.
2. Description of the Related Art
Developing devices for supplying developer to an electrostatic latent image on a photoconductor drum have been provided according to the prior art in image forming apparatuses such as printers which form images in a xerographic manner using a non-magnetic, one-component type developer.
This type of developing device is disclosed in Japanese Unexamined Patent Publication JPA 2-101485(1990), for example. The developing device 100 described in the publication comprises, as shown in FIG. 114, a sleeve 102 which rotates to supply developer 106 deposited on its periphery to a photoconductor drum 101; a developer feed roller 103 with recess sections on its periphery, placed near and in noncontact with the sleeve 102; a scrubbing member 105 for scrubbing developer 106 in the recess sections 104 on the periphery of the developer feed roller 103, etc. The developer 106 in the developing device 100 is loaded in the recess sections 104 on the periphery as the developer feed roller 103 is rotated and scrubbed by the scrubbing member 105 to be received in a developer-supplying chamber 107. The developer 106 received in the developer-supplying chamber 107 is then deposited on the sleeve 102 and then on the photoconductor drum 101 at the section 108 of contact with the photoconductor drum 101 as the sleeve 102 is rotated.
The publication also discloses other types of developer feed rollers with recess sections formed on the periphery, including a developer feed roller 110 having curved recess sections 111 as shown in FIG. 115, a developer feed roller 120 coated with a sponge 121 as shown in FIG. 116, and a developer feed roller 130 covered with bristles 131 as shown in FIG. 117, all of which are designed to scrub the developer 106 on the surfaces of the developer feed rollers 103, 110, 120 and 130 with a scrubbing member 105.
Other embodiments of this type of developing device include, for example, the one shown in FIG. 118 which is manufactured by Epson, Inc. This developing device is designed in such a manner that a circular developer feed roller 201 is rotated opposite to the direction of a developing roller 202, and a developer-scrubbing member 203 is provided at the bottom of the developer feed roller 201 to convey developer to the developing roller 202.
Xerography-based image forming apparatuses such as copiers and laser printers are equipped with a developing device for depositing developer comprising a coloring pigment on an electrostatic latent image formed on a photoconductor as a recording medium in order to visualize the electrostatic latent image. Here, the electrostatic latent image need not be black; replaceable developing devices which contain differently colored types of developer may be mounted in the body of an image forming apparatus for color printing.
Specifically, the dominating developing developer for color printing of the prior art is two-component type. This means that developer composed of a coloring pigment and a resin fails to produce a desired color if mixed with a magnetic material or another material.
Therefore, if one-component type color developer is desired, the one-component type developer is inevitably non-magnetic developer rather than magnetic developer. In cases where such non-magnetic developer is used, developing devices with a magnetic brush according to the prior art can no longer be used. This is because magnetic force fails to adsorb developer on the periphery of a developing roller in a given amount as controlled by a control member.
As a countermeasure, the above-mentioned developing device shown in FIG. 115 has been presented. The developing device 112 shown in the drawing is equipped with a developing roller 113 which conveys a given amount of one-component type developing developer 106, which is stored in a developer tank 114 supporting the developing roller 113 in a rotatable manner, to a photoconductor with an electrostatic latent image formed thereon to visualize the electrostatic latent image, and with a developer feed roller 110 for supplying the developer 106 to the developing roller 113 for its deposition thereon. Here, a controller member 115 is pressed against the periphery of the developing roller 113 in order to ensure that a constant amount of developer 106 is deposited on the developing roller 113. With this configuration, developer 106 supplied to the curved recess sections 111 of the feed roller 110 is forcedly brought into contact with the developing roller 113 for its deposition on the developing roller 113 as the developer 106 passes along the developing roller 113, leveled to a constant amount as it passes along the control member 115 halfway during conveyance by rotation of the developing roller 113, and then conveyed to the developing region facing the photoconductor.
With the above-mentioned prior art developing devices for image forming apparatuses, however, the developer feed rollers 103 and 110 shown in FIG. 114 and FIG. 115 tend to have different levels of torque in the case where the scrubbing member 105 scrubs developer 106 in the peripheral recess sections 104 or the curved recess sections 111 and in the case where the scrubbing member 105 scrubs developer 106 on the shoulder sections of the developer feed rollers 103 and 110. That is, a load on the developer feed rollers 103 and 110 during scrubbing is larger at the shoulder sections than at the peripheral recess sections 104 or the curved recess sections 111. As a result, this difference in torque has the adverse effect of influencing rotation of the motors which drive the developer feed rollers 103 and 110, e.g., change in the driving force, thus resulting in the problem of irregular image quality.
In addition, with the developer feed roller 120 shown in FIG. 116, the scrubbing member 105 must dig into a sponge 121 when it is attempted to scrub the developer 106 caught in the sponge 121, and this also results in increased torque of the developer feed roller 120. Further, the sponge-coated roller is more expensive than resin rollers.
Furthermore, with the sponge-coated roller, the developer 106 once scrubbed from the sponge 121 tends to be caught by the sponge 121, and this results in lower capacity to convey developer 106.
On the other hand, since the developer feed roller 130 with bristles 131 which is shown in FIG. 117 requires an increased amount of digging by the scrubbing member 105, torque of the developer feed roller 130 increases.
Further, in the same manner as with the developer feed roller 120 coated with the sponge 121, the developer 106 once scrubbed from the bristles 131 tends to be caught again by the bristles 13, and this results in lower capacity to convey developer 160 as well. An additional problem is the increased cost of the materials.
A problem with the circular developer feed roller 201 shown in FIG. 118 is that its capacity to move developer 106 by its rotation is low, and this results in lower capacity to supply the developer 106. This problem causes a further problem of creation of sections free of developer 106 on the developing roller 202 after solid printing, and this increases the probability of white void in the solid printing.
These problems are particularly serious for compact printers. This is because compact printers, which use a downsized motor for miniaturization and cost-reduction, have a problem in that the developer feed roller 103 and other members constructed as described above tend to undergo change in torque and are therefore irregularly driven for rotation, resulting in the formation of images with unfavorable quality.
As a countermeasure against the problems mentioned above, for prevention of leakage of developers from developing devices before use, Japanese Unexamined Patent Publication JPA 58-107560 (1983) discloses a structure shown in FIGS. 119A and 119B, wherein the region of a developer storage section 301 is separated from the region of a developing roller 302 by a sheet-like sealing member 303 when in a not-yet used state so as to prevent leakage of the developer sealed in the developer storage section 301 through the developing roller 302 during transportation, which contaminates the surroundings, and just before use the sheet-like sealing member 303 is opened by an opening member 304 to provide double doors through which the developer is supplied to the region of the developing roller 302. This structure, however, is complicated and increases the cost.
On the other hand, with the prior art developing device 112 shown in FIG. 115, it is extremely difficult to level developer 106 by the control member 115 as the developer 106 passes along the member 115 on the way to the point of development. More specifically, the requirement of uniform pressing of the control member 115 against the developing roller 113 along the full axial length thereof is rather difficult to accomplish because adjustment of pressure for the uniform pressing is extremely troublesome, and this results in distribution of irregular axial force of pressing which in turn results in irregular amounts of deposition (amounts of application) of developer 106. Additionally, it is also extremely difficult to provide a smooth and even section of contact between the control member 115 and the developing roller 113. Therefore, formation of irregularities on the section of contact with the developing roller 113 may impair evenness of application. Conversely, if the control member 115 has an excessively smooth surface, the developing roller 113 may have increased torque.
In addition, even if developer 106 may be applied uniformly with the aid of the control member 115, repeated development tends to cause partially thinner density and gradual reduction in line width along the direction of paper conveyance when halftone images or solid images are developed. This tendency becomes more significant as the amount of developer 106 remaining in the developer tank decreases. Should this tendency further increase, torque of the developing device, particularly torque for rotating the developing roller 113 increases, and consequently there will occur a problem in that the developing roller is put out of rotation, or locked.
This is caused by gradual fusion of developer 106, which is controlled by the control member 115, to the control member 115 by pressure from the control member 115 and the developing roller 113. As a result, since the fused sections are elevated to provide irregularities (bumps and dents) at the section of the control member 115 which contacts the developing roller 113, and the amount of developer applied to the developing roller 113 becomes irregular accordingly in the peripheral direction along the irregular surface, that is, it is not consistent in the axial direction, and this results in the above-mentioned formation of streaks on the paper in the direction of conveyance. The fused developer 106 also increases the torque of the developing roller 113.
On the other hand, with the prior art developing device shown in FIG. 115, for deposition of developer 106 supplied by the feed roller 110 on the developing roller 113, the developer 106 passes along the developing roller 113, and the developer pressure in the developer-supplying chamber 107 is increased to promote deposition of the developer on the developing roller 113. In cases where the developer pressure in the supplying chamber 107 is not consistent due to the relationship between the feed roller 110 and the developing roller 113, the amount of charge of the developer 106 by friction is not stable, and thus since the pressure of the developer 106 to be deposited on the developing roller 113 is not constant, the amount of developer 106 deposited on the developing roller 113 is not stable.
With the developing device 112 shown in FIG. 115, the location of the scrubbing member 105 for scrubbing developer 106 remaining in the curved recess sections 111 of the feed roller 110 which supply developer 106 changes as the developer feed roller 110 rotates. For this reason, the volume of the developer-supplying chamber 107 changes, and thus the pressure of developer 106 at that location cannot be constant, as mentioned above. Accordingly, not only is the developer 106 inconsistently charged by friction, but there also remains the problem of the notable influence of the developing roller 113 on the amount of application of developer 106.
Although the foregoing description is centered on the amount of non-magnetic, one-component type developer 106 deposited on the developing roller 113, magnetic, one-component type developer 106 also must be deposited on the developing roller 113 in a constant amount, and for this the control member 115 is provided in the developer tank 114 as in the case with non-magnetic, one-component type developer. Even with this control member 115, since developer 106 is pressed against the developing roller 113 to control the amount of developer 106 to be deposited, the developer 106 is deposited on the control member 115, repeating the deposition causes fusion of the deposited developer and the inconvenience explained above occurs in the same manner.